The present invention is related to storage tanks and system used for containing liquids having volatile organic compounds.
More particularly, the present invention is related to access hatches for inspecting and sampling the liquids contained within a storage tank.
A storage tank (referred to alternatively throughout as a tank, stock tank or storage tank) for containing liquids may be of any type, terrestrial, marine, rail or truck and constructed of virtually any industrial material, steel, fiberglass and plastic being the most common. Storage tanks intended for containing liquid hydrocarbons (oil, crude oil, refined products, drip gas, etc.) that may produce volatile organic compounds (VOCs) are typically constructed of steel or fiberglass.
Most tanks exhibit generic design features, such as that depicted in FIG. 1. These include the tank exterior shell 101 having continuous sides, floor and roof (some designs employ floating external or internal roofs) for containing liquid 110 in its interior, while preventing harmful VOCs 114 from being vented into the atmosphere. Liquids 110 are piped into the interior of tank 100 through inlet pipe 106 and drained from tank 100 through outlet pipe 108. Liquid 110, most often a hydrocarbon based fluid, such as crude oil and refined products, emit VOCs 114 into the tank volume above the surface 112. One or more VOC recovery systems 105 recovery VOCs 114 as they are emitted from fluid 110. VOCs 114 exit tank 100 from a pipe opening in the roof (which may be stationary, floating or have both) and into a closed conduit connected to a recovery apparatus of some type. Additionally, emergency vent 104 is provided on the roof for venting VOCs 114 into the VOC recovery system (or the atmosphere as shown) for the case of a rapid buildup of pressure within the interior tank 100 not handled by VOC recovery system 105. Finally, disposed on the roof of tank 100 is a manhole or access hatch, enabling tank operators to gain access to liquid 110. Generically, these hatches are known in the petroleum industry as “thief hatches” because they enable tank operators to visually inspect and gauge the contents of tank 100, as well as “thief” or sample the liquid stored within.
The EPA has identified the thief hatch misuse as a major contributor for the unchecked release of VOCs into the atmosphere. In fact, several state environmental agencies have dubbed thief hatches as the weakest link of VOC control and mandated specific thief hatch monitoring and management. Misuse occurs in a number of ways. First, obviously, by tank operators leaving the thief hatch open following thiefing operations. Second, and much less conspicuous, is by leaving the thief hatch closed, but unlatched. Generally, unless “dead weighted”, thief hatches are relatively light weight and will not substantially reduce the flow of VOCs into the atmosphere unless the lid is securely latched to its base.
Another form of hatch misuse is related to thief hatch maintenance. Poor maintenance practices and standards are a major contributing factor to VOC leakage. While leaks can occur due to sealing ring wear or failure on the base flange and/or hatch cover, vacuum plate damage, pressure or vacuum compensation spring failure or valve stem or guide wear, by far the most common maintenance related failure involves the pressure or cover gasket (described hereinafter as a pressure gasket, lid cover gasket of merely a cover gasket). The atmospheric seal for a thief hatch for use in the petrochemical industry is provided by a pliable or semi-pliable cover gasket or seal. Typically, cover gaskets for thief hatches are comprised of neoprene, Nitrile, BUNA-N (nitrile) rubber, EPDM rubber (ethylene propylene diene monomer (M-class) rubber), ECH (Epichlorohydrin) or Viton® (Viton® is a registered trademark and tradename of E.I. DuPont de Nemours and Company, Inc. of Wilmington, Del.). However, other materials may be used as gasket materials depending on the demands of the respective usage, these include, but are not limited to open and closed cell foam and sponge, natural rubber, BUNA-N (nitrile) rubber, BUNA-S rubber, ECH (epichlorohydrin), neoprene rubber, neoprene coated fabrics, EPDM rubber, EPDM sponge, silicone rubber, EVA (ethylene vinyl acetate), polyethylene, polystyrene, polypropylene, polyurethane, polyimide, PVC sponge, vinyl—flexible & rigid, bucote, nicote, steel, stainless steel, cork, cork-rubber compositions, plastics, cloth, cloth inserted rubber and coated fabrics. Due to the design of most thief hatches, the gasket material must have a very low hardness. The quantification of gasket hardness, durometer, is an exceptionally complicated subject and will not be discussed herein, except to mention that the hardness of elastomeric gasket materials is usually described by two parameters: Shore hardness and compression force deflection (CFD). Shore hardness in this context (Shore A) is a measure of how well a material resists a permanent indentation. CFD is a measure of firmness as defined by ASTM standard D1056, as the force necessary to reduce the thickness of a material by 25%). Both Shore A and CFD for thief hatch gasket materials must be low in order to provide a tight seal with relatively low closure force provided by the pressure compensation spring in the pressure/vacuum compensation valve (the design and function of the pressure/vacuum compensation valve system will be discussed below with regard to FIGS. 2 and 3).
In any case, good thief hatch maintenance requires the thief hatch gasket be in good physical repair and the gasket and corresponding upper and lower sealing rings be free of any contaminants that may inhibit proper sealing. Thief hatch maintenance is difficult to qualify because often the appearance of the gasket and corresponding upper and lower sealing rings is not noteworthy. However, repeated thiefing and gauging operations contribute to the degradation of the gasket and sealing rings. Typically, only a single field gauger climbs onto the tank's roof with a plumb bob, line, sampling thief, thermometer, sample containers, notepad and other tools as needed. The gauger opens that lid of thief hatch 102, drops the plumb bob and checks, verifies and records the fluid level. Next, temperatures are recorded from various depths. Finally, the gauger thiefs samples of fluid from various depth and transfers the samples to corresponding sample containers and records the samples' information. All too often the gauger retrieves the final thief sample, slams the thief hatch lid and latches it with his foot while simultaneously transferring the fluid sample from the thief to the container. Field gaugers travel from one tank to another, pad to pad, and site to site in a rather hectic fashion, gauging/thiefing different tanks at different schedules. Consequently, thief hatch gaskets become worn, torn or deformed from repeated hard lid closings and from contaminates that accumulate on the thief hatch gasket and corresponding lower and upper sealing rings from spillage from line, gauging and thiefing operations, as well as from merely opening and closing the hatch cover as will be discussed below. All of the above contribute to VOC leakage at the pressure gasket.
Improper thief hatch operating has become such an issue that several states now require that only trained field gaugers open thief hatches and that they follow a strict protocol of: verify absence of VOCs; unlatch the thief hatch lid; inspecting lid, valve, gasket and upper and lower sealing rings; perform gauging/thiefing operations; re-inspect lid, valve, gasket and upper and lower sealing rings; cleaning lid, valve, gasket and upper and lower sealing rings; replace thief hatch and valve gaskets as needed; close lid; latch thief hatch lid, verify full engagement of the thief hatch latch and verify absence of VOCs.
FIGS. 2A, 2B, 2C and 2D are oblique, side, top and cross-sectional views of a diagram for a generic thief hatch. Typically, thief hatches 102 comprise four basic components: lid 202 and hatch cover 230 assembly (including upper sealing ring 232 and pressure/vacuum compensation valve mechanism 234), flange base 220 and lower sealing ring 224 assembly, hinge 208 assembly with optional latch 204 with catch 206. Disposed along flange base 220 of thief hatch 102 is a plurality of bolt holes for receiving bolts or studs 226 from the roof of tank 100 over flange gasket 222 (threaded nuts are used in conjunction with studs). Lid 202 and hatch cover casting 230 are two independent assemblies joined together with the pressure/vacuum compensation valve 234, the function of which will be described further below. Lid 202 and hatch cover casting 230 pivot horizontally from flange base 220 via hinge 208 about hinge pin 209 (as can be seen in FIG. 3A). In practice, one leaf of hinge 208 is secured to lid 202, as is catch 206, while the second leaf is secured to flange base 220 and the leaves are hingedly coupled with hinge pin 209.
Generally, in the closed position, lid 202 completely covers flange base 220. Also, upper sealing ring 232 of hatch cover 230 engages lower sealing ring 224 of flange base 220, with lid pressure gasket 210 interposed between the two to provide an air-tight seal. Lid pressure gasket 210 is positioned adjacent to upper sealing ring 232 on hatch cover casting 230 and secured by friction to vertical lip 233 of hatch cover casting 230. Pressure gasket 210 is most often a circular shaped flat seal comprised of a pliable or semi-pliable material as described above. Optionally, lid pressure gasket 210 may have a “U”-shape cross-sectional shape that fits snuggly over the outer edge of upper sealing ring 232 of hatch cover casting 230 (not shown in the figures). While in the closed position, optional latch 204 pivots about latch pin 205 and onto catch 206, thereby latching lid 202 to base flange 220.
In the latched position, latch 204 secures lid 202 to base flange 220 through catch 206 at a force predetermined by pressure compensation spring 235 within pressure/vacuum compensation valve 234, discussed immediately below. Maintaining a constant uniform pressure on lid pressure gasket 210 from hatch cover casting 230 toward lower sealing ring 224 on flange base 220 is key to sealing VOCs within tank 100. Here it should be mentioned that not all thief hatch designs utilize a latching mechanism, some use dead weight of lid 202 for exerting sealing force (pressure) (not shown in the figures). In addition, latch 204 and catch 206 assembly depicted in the figures are merely one exemplary latch, other latch designs employ a “J”-hook, and/or J-hook and cam mechanism or various types of levered cams for amplifying a force between hatch cover casting 230 and flange base 220 (also not shown in the figures).
The force (pressure) exerted on lid pressure gasket 210 from hatch cover casting 230 is predetermined by pressure compensation spring 235 inside pressure/vacuum compensation valve 234. Hatch cover 230 is movably secured to lid 202 through pressure/vacuum compensation valve 234. Pressure/vacuum compensation valve 234 is a means for compensating the internal pressure of tank 100 to near that of the atmosphere to avoid rupturing or imploding the tank's exterior shell 101 i.e., the tank's walls and roof). Pressure/vacuum compensation valve 234 is cylindrically-shaped and generally comprises a pair of springs which enable each of lid 202 and vacuum plate 239 to move (open) independently of one another in order to compensate for pressure differences between the interior of tank 100 and the atmosphere.
Pressure compensating spring 235, disposed within pressure/vacuum compensation valve 234, forces lid 202 and hatch cover casting 230 apart. In closing lid 202 onto base flange 220, pressure compensating spring 235 is compressed. This exerts a predetermined downward force on lid 202, thereby squeezing lid pressure gasket 210 between the upper surface of lower sealing ring 224 on flange base 220 and the lower surface of upper sealing ring 232 on hatch cover casting 230. Consequently, the magnitude of the sealing force across pressure gasket 210 is determined by the compressive strength of pressure compensating spring 235. In the case of internal pressure within tank 100 increasing, the pressure creates an upward force on the lower surface of hatch cover casting 230 (and vacuum compensation plate 239). Once that upward force exceeds the sealing force created by pressure compensating spring 235, hatch cover casting 230 moves upward relative to lid 202, thereby breaking the seal of pressure gasket 210 and allowing vapors from tank 100 to escape into the atmosphere through lid cover opening 212. Dead weight thief hatches do not use pressure compensating valves for controlling the interior pressure of tank 100, but instead use the dead weight of the lid to provide a predetermined level of sealing force.
Vacuum compensating spring 236 is a compression spring disposed within pressure/vacuum compensation valve 234 and forces vacuum plate 239 and hatch cover casting 230 together. The magnitude of the force is predetermined by the compressive force of vacuum compensating spring 236. In case of a vacuum occurring within tank 100, the atmospheric pressure is greater than the internal pressure of the tank. When the force on the upper surface of vacuum plate 239 (created by the atmospheric pressure reaching vacuum plate 239 through vacuum compensation ports 237 and lid cover opening 212) exceeds the combined force of vacuum compensating spring 236 and the force on the lower surface of forces vacuum plate 239 created by the internal pressure of tank 100, vacuum plate 239 moves downward relative to lid 202, thereby breaking the seal of vacuum plate seal 240 and air from the atmosphere to enter tank 100 through vacuum compensation ports 237 and from lid cover opening 212. Some dead weight thief designs make use of a vacuum compensating valve to compensate for vacuum conditions within tank 100.
With further regard to latching thief hatches with regard to VOC leakage, such as those exemplary hatches depicted in the figures, these hatches may be understood to operate in three distinct positional or closure states. FIGS. 3A, 3B, and 3C are diagrams depicting the three closure states of a typical thief hatch: open in FIG. 3A; closed in FIG. 3B and closed and latched in FIG. 3C. One could conclude from the figures that thief hatch 102 of FIG. 3B in the closed position and of FIG. 3C in the closed and latched position are remarkably similar. However, with some hatch designs (and depending on the compressive strength of pressure compensating spring 235) lid 202 will be slightly skewed from flange base 220, as is apparent from the position of lid 202 in FIG. 3B. This is often very slight and virtually impossible to recognize from any vantage point except a direct side view. However, as a practical matter, in the closed (but not latched position) these hatches are actually open and not sealed. Only a small portion of pressure gasket 210 is actually in contact with lower sealing ring 224 of base flange 220, leaving the remaining circumference of pressure gasket 210 unsealed and open to the atmosphere.
FIGS. 3D and 3E are cross sectional views of thief hatch 102 with lid 202 closing (or opening) onto base flange 220 which better depicts the movement of pressure/vacuum compensation valve 234 during opening and closing of lid 202. By design, the lowermost portion of pressure/vacuum compensation valve 234, including vacuum plate 239, extends beyond the lower edge of lid 202 whenever lid 202 is closed as can be seen in FIG. 2D, where thief hatch 202 is depicted in the closed position. However, in the open position, upper sealing ring 232, lid pressure gasket 210 and vertical lip 233 of hatch cover 230 all extend beyond the lower edge of lid 202 as can be understood by comparing thief hatch 202 depicted in FIG. 2D, in the closed position, with thief hatch 202 depicted in FIG. 3A, with thief hatch 202 in the open position.
More importantly for understanding the need for the present invention, as lid 202 closes about hinge 208, initially lid pressure gasket 210 contacts lower sealing ring 224 of base flange 220 only at a point nearest to hinge 208, see FIG. 3D. At that position, pressure compensation spring 235 is fully extended. As lid 202 continues to close about hinge 208, lid pressure gasket 210 continues its bias against lower sealing ring 224, which gradually compresses pressure compensation spring 235 (compare pressure compensation spring 235 in FIG. 3D with that in FIG. 3E). Importantly, the lower surface of lid pressure gasket 210 scrapes along the upper surface of lower sealing ring 224 while exerting closing force to pressure/vacuum compensation valve 234 until pressure/vacuum compensation valve 234 is in its uppermost position see FIG. 2D and lid 202 is fully closed. This closing process takes a toll on lid pressure gasket 210. To be sure, the above-described phenomenon is not uncommon occurrence with hatches or other types of doors with seals, such as home entry doors. In certain critical applications, such as pressurized aircraft fuselage doors, the hatch uses an interlocking hinge system that first allows the hatch to swing into the closing position without the door seal contacting the frame, and then the hinges, hatch and door seal move parallel to and mates with the frame in unison, thereby eliminating friction on the door seal.
However, with regard to the thief hatch applications, stress on the portion of lid pressure gasket 210 closest to hinge 208 is even more pronounced because that part of lid pressure gasket 210 is the support member that absorbs all of the biasing force necessary to compress pressure compensation spring 235 through the range of closing angles, compare lid 202 in FIGS. 3A, 3D, 3E, and 2D. The closing/opening forces have even a more dramatic wear effect on gasket 210 due to the low durometer of the gaskets used in thief valves. Additionally, as is apparent from the figures, the moving components of pressure/vacuum compensation valve 234 are subject to accelerated wear because the closing force is not applied parallel to the movement direction of hatch cover 230. Consequently, aside from the inherent problems associated with closing and latching a thief hatch, VOC leakage from mechanical failure is also very prevalent.
Although VOC leakage from mechanical failures happens, the far more pervasive, as well as correctable problems, are hatch closure issues. Clearly, operators can easily mistake a closed thief hatch for a closed and latched thief hatch (as demonstrated by the views in the previous figures). It is often even more difficult to distinguish merely closed from closed and latched with other latch designs, such as those where the latch extends downward from the thief hatch lid, because pressure gasket 201 is not fully in place with lid 202 merely closed. For those designs, the latch automatically falls into a nearly latched position by closing the hatch, often an operator must wiggle the latch to determine whether or not it is actually engaged with the latch catch. Very importantly, however, a closed but not latched thief hatch can leak VOCs at substantially at same rate as that of a thief hatch in the open position. Hence, even though a closed thief hatch appears similar to a closed and latched thief hatch, for the purposes of VOC abatement, a closed thief hatch is actually much more similar in operation to an open thief hatch. Consequently, recent efforts have been made to verify that a thief hatch is actually truly latched and not merely closed.
The prior art has dealt with the problem of securing thief hatches by employing various sensors to detect the position of the thief hatch lid. The detector is often connected to a light or audible alarm for alerting operators when the thief hatch lid is open. For example, an adjustable, normally closed (momentary off) door plunger switch 402, as depicted in FIG. 4A closes the alarm circuit when the thief hatch lid is open. Plunger switch 402 comprised mounting hardware (not shown), body 404 containing the switch contacts and spring, plunger 408 and plunger adjustment 406 for adjusting the vertical height of plunger 408 in the electrically open state. Typically, plunger switch 402 mounts to flange base 220 directly below an edge of thief lid 202 and height of plunger 408 is adjusted to the electrically open state with thief hatch lid 202 fully closed as depicted in FIG. 4B, but electrically open state with thief hatch lid 202 in any state except fully closed, see, for example, FIG. 4C.
Adjusting the correct operating height of plunger 408 is critical to its proper operation. Additionally, for maximum effectiveness, plunger switch 402 must be located opposite the hatch hinges, near latch 204. In this position, plunger switch 402 is highly suitable to being jolted and bumped during gauging and thiefing operations. In any case, while this prior art mechanism will alert the operator to an open thief hatch, it generally cannot distinguish between a thief hatch that is merely closed, from one that is completely latched.
What was needed was a sensor that detects the position of thief hatch latch 204, not merely the position of thief hatch lid 202. One mechanism for sensing the proper latch position known in the prior art is, for example, a normally open reed switch 502, as depicted in FIG. 5A. Reed switch 502 comprised mounting hardware (not shown), body 504 containing reed switch contacts (also not shown) with a ferric arm attached to one reed contact and magnet 506. Whenever the ferric arm inside body 504 is proximate to magnet 506, the magnet attracts the ferric arm, causing the attached reed contact to close on a second internal contact, thus completing an electrical circuit. Typically, reed switch body 504 is mounted to flange base 220 in a position proximate to the position of thief hatch latch 204 when fully engaged with latch catch 206, see FIGS. 5B and 5C. Magnet 506 is then attached to thief hatch latch 204 at a position closest to body 504 with thief hatch latch 204 fully engaged with latch catch 206. Here, rather than the switch circuit closing with the position of thief hatch lid 202, the reed contacts close the alarm circuit whenever magnet 506 attached to thief hatch latch 204 is proximate to body 504, that is whenever thief hatch latch 204 engages latch catch 206.
Other reed switch sensors for “J” latch designs use a pair of pins, one for catching and the other as a hinge pin for the J hook. With those, the pins are modified, the catch with an internal magnet and the hinge pin for the J hook is supplied with a reed switch as the one described above. This type “J” latch is easily modified for a lid position sensor by merely replacing the original catch and J hook hinge pin with the modified components.
This type of sensor overcomes the shortcoming of the prior art sensor's inability to distinguish between a closed thief hatch and a closed and latched thief hatch. However, reed switch 502 only senses the position of thief hatch latch 204. It is possible for the latch's position to mimic the fully engaged position without thief hatch lid 202 being fully latched or in some cases, even being closed. Furthermore, reed switch 502 is also mounted at the latch in the front, at the working area of the thief hatch, thereby also being suitable to being jolted and bumped during gauging and thiefing operations. Furthermore, no prior art mechanism provides any information as to the mechanical condition of the pressure seal.
What is needed is a mechanism for correctly detecting the closure state of thief hatch lid 202, one which can accurately distinguish between a thief hatch lid that is closed and latched from one that is merely closed. Additionally, a mechanism that can also be used to determine baseline parameter values for a correctly functioning thief hatch would be very advantageous. The values of those parameters could then be monitored and compared to the baseline values of the parameters to determine the condition of the thief hatch. The greater the deviation of the monitored values from the baseline parameter values, the greater the risk of VOC leakage from the thief hatch.